The invention relates to a method for detecting accidental arcs on a cable, especially on a cable of an on-board electrical system of an aircraft or spaceship. It further relates to a device for carrying out the method. The term accidental arc relates hereby in particular to the so-called arc tracking.
The problem of the so-called “cable arc tracking” is known for many years in particular in electrical systems of aircrafts and/or spaceships. Arc tracking involves the establishment of a conducting connection in a bunched cable between two neighboring cables as a consequence of, e.g., damages and/or contaminations (wet arc tracking) or scraping on a metallic edge (dry arc tracking). This connection can be initially highly resistive (wet arc tracking) or may also be directly low resistive (dry arc tracking). Wet arc tracking designates hereby between two defects in insulation a conductive connection which is made by an electrolyte, e.g., a cleaning liquid, and attacks the cable insulation. In conjunction with arc formation, the result is oftentimes a thermal destruction of the entire bunched cable along a certain length. In contrast thereto, a metallic conductor produces the conducting connection in dry arc tracking with relatively low resistance, and the arc is practically established instantly. This may also lead to the thermal destruction of the bunched cable.
The destruction of the bunched cable per se is not the only problem here. Rather, consequential damages are critical as a result of surrounding material being set on fire. Such cable fires lead to ensuing explosion-like faults, which may be the cause for catastrophic damages to the aircraft or spaceship.
When investigating the causes of occurring accidental arcs or arc tracking in cables of such electrical systems, it could be demonstrated that the resistance capability of cable insulations against arc tracking depends exclusively on the materials used for the cable sheath, on the one hand. On the other hand, the material (POLYIMIDE) used heretofore is characterized by a very high water absorption so that damage as a result of cracking in the insulation, especially during frequent temperature changes as occurring during flight operation, is promoted. Furthermore, the thermal destruction results in a formation of carbonized deposits at about 800° C. and thus enables the arc tracking, i.e. a further migration of the arc along the cable harness. In contrast thereto, materials such as, for example, POLYTETRAFLUOROETHYLENE (PTFE) are relatively resistant against arc tracking.
Since the insulation of the cable types used in aviation are further made of POLYIMIDE film, all efforts undertaken so far in this field are directed to develop tests which allow a substantially unambiguous classification of cables as far as their resistance capability against arc tracking is concerned. Hereby, POLYIMIDE has not been used so far as component of cable sheathing because of its superior dielectric values and its high temperature resistance. Still, despite optimization of the processes and application of highest care during manufacture and laying of the bunched cable, accidents could not entirely be prevented as a result of cable arc tracking. Moreover, it has been recognized that flashover can occur in plug-type connectors under the influence of moisture and dirt, leading in the most unfavorable case to an ignition of neighboring parts.
The installation of protective measures is complicated by the fact that the amplitude of the current flowing during the arc tracking process, in particular during wet arc tracking, can lie significantly below the nominal current of typically installed protective overcurrent devices so that the latter can no longer be tripped. In contrast thereto, the amplitude values during dry arc tracking are higher as known per se. They occur, however, typically in intervals so that the converted energy is oftentimes insufficient to trigger the installed protective members.
To overcome the described problem, it is therefore desirable to provide an additional protection against accidental arcs in addition to the already existing conventional overcurrent protection in the electrical system of such an aircraft in order to protect the existing on-board installations as reliable as possible.